Update libpng to version 1.6.3

Change-Id: I86cb46ec376eb41324b82d2875757aeda734c5c1
diff --git a/tests/pngunknown-sAPI b/tests/pngunknown-sAPI
new file mode 100755
index 0000000..e087ef2
--- /dev/null
+++ b/tests/pngunknown-sAPI
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec ./pngunknown bKGD=save cHRM=save gAMA=save all=discard iCCP=save sBIT=save sRGB=save "${srcdir}/pngtest.png"